In recent years, light emitting diodes (LEDs) capable of emitting light with high luminance and high illuminating efficiency have been developed. In comparison with a common incandescent light, a LED has lower power consumption, long service life, and quick response speed. With the maturity of the LED technology, LEDs will replace all conventional lighting facilities. Until now, LEDs are widely used in many aspects of daily lives, such as automobile lighting devices, handheld lighting devices, backlight sources for LCD panels, traffic lights, indicator board displays, and the like.
For increasing the overall brightness values, a plurality of LEDs are connected in series to form a LED string. Due to the fabricating processes, the initiating voltages of different LEDs are somewhat distinguished. Generally, the initiating voltage of respective LED is ranged between 3.2V and 3.6V. That is, the initiating voltage of a specified LED string falls into a specified range. For example, a LED string consisting of thirty serially-connected LEDs has an initiating voltage in the range of between 96V and 108V.
Generally, the LED string is connected to a power supply circuit. The power supply circuit is used to drive illumination of the LED string. Since the initiating voltage of the LED string is in a specified range, a rear-stage converting circuit of the power supply circuit will receive an input voltage from power source (e.g. a utility source) and convert the input voltage into the initiating voltage required for driving illumination of the LED string. By adjusting the initiating voltage, the current passing through the LED string is controlled to a constant value and thus uniform brightness is obtained.
As the number of LEDs contained in the LED string is increased, the input voltage is converted into higher voltage-level driving voltage by the rear-stage converting circuit. Since the input voltage is converted into the higher voltage level, the rear-stage converting circuit has a high power conversion loss and the operating efficiency is impaired. In addition, the rear-stage converting circuit should contain high pressure-resistant components, and thus the conventional power supply circuit is not cost-effective.
For obviating the drawbacks encountered from the prior art, there is a need of providing a power supply circuit for powering LEDs with reduced power conversion loss.